1. Field of the Invention
The present invention relates in general to the manufacture of heterojunction bipolar transistors (HBT). More particularly, it relates to a novel material system and epitaxial structure for a single heterojunction bipolar transistor (SHBT) having at least one AlPSb/InP layer and an InP substrate, and capable of being used in high-speed high-power applications.
2. Description of the Related Art
A HBT is a known type of semiconductor transistor employing an emitter made from a wide-gap semiconductor, a base made from a narrow-gap semiconductor, and a collector made from either a narrow bandgap or wide bandgap semiconductor. High-speed, high-power HBTs are used in a variety of electronic devices such as power amplifiers, low-noise amplifiers, and high-frequency switches.
One particular type of HBT is the double heterojunction bipolar transistor (DHBT). A DHBT is usually intended for power applications, because the collector is a wide bandgap semiconductor. A conventional DHBT (NPN-type) might include a Si-doped AlInAs layer as its wide-gap N-emitter, a Be- or C-doped GaInAs layer as its P-base, a lightly Si-doped InP layer as its N-collector, an InP substrate, a first grading layer between the base and emitter (B/E grading layer), and a second grading layer between the base and collector (B/C grading layer). The high-power capabilities of the typical DHBT are provided by the InP collector since it provides a much higher breakdown voltage than an HBT with a narrow bandgap semiconductor. For high-speed, high-power applications, the use of InP as a collector provides high saturation velocity, large bandgap, and large breakdown voltages. The high saturation velocity translates into a short collector transit time, and the large bandgap and large breakdown field translates into high breakdown voltages, BV.sub.CBO and BV.sub.CEO.
Although DHBTs provide suitable power performance, they have the disadvantage of containing conduction band discontinuity between the GaInAs base layer and the InP collector layer (B/C interface) which blocks the flow of electrons from the base to the collector resulting in poor output characteristics. For example, the conduction band discontinuity at the B/C interface of a GaInAs base and a InP collector is about 0.25 eV for gas-source molecular beam epitaxy grown materials. This necessitates the use of several elaborate grading layers and bandgap engineering between the GaInAs base and the InP collector, both of which are difficult to implement. Additionally, because AlInAs, a conventional emitter, has about the same bandgap as InP, AlInAs could not be used as an emitter in a DHBT employing InP as the base and the collector.
Another type of HBT, known as a single heterojunction bipolar transistor (SHBT), avoids the above-described problems by not utilizing InP in its collector. However, this results in a SHBT construction that provides high-speed but does not typically also provide the high-power capabilities that result from using InP as the collector layer. For example, a conventional SHBT may employ Si-doped AlInAs as the wide-gap N-emitter, Be-doped GaInAs as the P-base, Si-doped GaInAs as the N-collector, an InP substrate, and grading layer between the emitter and base. Because the intrinsic speed of the transistor is determined by the transit time through the base and collector, a high-speed transistor employs materials with high electron mobility and saturation velocity in both the base and the collector. GaInAs is a very high-speed material because its bandgap is low. However, GaInAs is a low breakdown voltage material, so transistors employing GaInAs as a collector cannot deliver high-power performance.
Thus, it would be beneficial to provide a material system and epitaxial structure for a SHBT that allows it to incorporate InP in both the base and collector, thereby allowing the SHBT to provide both high-speed and high-power capabilities. At the same time, an emitter with a bandgap larger than InP is desirable to improve the emitter efficiency and the current gain. In addition, an emitter employing a chemically stable material lattice-matched to InP is desirable for ease of growth.